The present invention relates to a numerically controlled machine tool adapted to machine a workpiece to a desired shape by controlling the movement of the position of the tip of a tool from a predetermined processing or machining reference point, in accordance with numerical data.
Conventionally, the numerical control of machine tools has been considered to control them by putting in commands in a coded form of the path to be taken by the tip of the tool during machining. In conventional numerical control wherein as means for putting numerical data into the control section, coded commands are put in by a typewriter or punched tapes having coded commands put therein are used, it is necessary as a matter of course to prepare a program having details of the machining process coded therein, but such preparation of program, as is known, involves much difficulty. Thus, it requires a programmer who has acquired special techniques, and the procedure for preparation of a program comprises the steps of reading the design drawings to decide the necessary process, order of machining, tool to be used, direction of feed, feed rate, quick feed position, cutting feed starting position, amount of machining, spindle rpm, tool exchange command, etc., preparing a coding sheet having this data written thereon in the predetermined order of description by using such automatic programming language as APT or EXAPT, putting data based on said sheet into a computer or an automatic tape preparing machine (very expensive) to prepare an NC tape or manually calculating the tool path necessary for machining, preparing a program sheet having details of the result described therein in NC language in accordance with a given format, and handing it to the key puncher who then prepares a punched tape on the basis of it or operating the operating panel for direct insertion of the data rather than preparing a punched tape having the contents of said program sheet recorded therein. This procedure involves much time and labor. Moreover, since such tape prepared for a particular type of machining can only be used for that machining, there is an inconvenience that if the machining shape is changed even slightly, the tape can no longer be employed as it is.
U.S. Pat. No. 4,033,206 granted to the assignee of the instant application discloses an NC machine tool which requires no such tapes whatsoever and is characterized in that the working modes of cutting tools are classified according to fixed cycles and control information is put into the machine through digital switches. A brief reference will now be made to said patent. Where it is desired to machine a blank to an intended shape, the final shape is a combination of some simple unit or basic forms. In other words, the machining procedure for obtaining the final shape is a combination of some simple basic machining operations, such as outer diameter cutting, inner diameter cutting, facing, tapering and grooving. Although the path of actual movement of the tip of the tool (or the amount and direction of movement of the tool tip) differs with individual machining operations, the movement of the tool tip in each basic machining operation is repetitions of a cycle having a definite characteristic pattern. If, therefore, data on the patternized movement of the tool corresponding to the unit shapes is stored in the memory of the control unit, the setting of a particular basic machining operation can be completed simply by designating the identification sign associated with the basic machining shape by the use of digital switches and then designating the finish dimensions of the associated portion of the workpiece by the use of digital switches. If such settings associated with required different basic machining operations are made in order and the start button is pushed, the machine tool will compute the paths of movement of the tool in the designated order on the basis of the designated finish dimensions according to the respective machining cycles and automatically perform all machining by repeatedly moving the tool until the desired final shape is obtained. In this case, however, the greater the number of machining steps, the greater the number of digital switches required. Another problem is that the manual operation determined by the contents of the process and by the corresponding identification signs can become complicated.
In addition, there is a system called MDI (manual data input) which seems analogous to the above described system in that control data is directly fed into the control unit without using tapes. This system, however, only has the function of a typewriter for preparing tapes and a memory unit for storing the data fed in by the typewriter and, in itself, is no different from a system which directly puts in tapes. Thus, the MDI system requires exactly the same keyboard operation as in the case of preparing a program sheet for the path of movement of the tool and preparing a tape on the basis of said program sheet. It would be much more efficient to have the key puncher prepare tapes at the office than to have the operator feed in the necessary data at the machine shop. Normally, this system is used only for auxiliary purposes, such as correction of tape preparation errors.
Since NC machine tools process work to a desired shape by controlling the amount of movement of the tool tip position from a predetermined machining reference point inherent in the machine, as described above, it is necessary to accurately establish the relative positional relation between the tool tip position and the machining reference point. The usual practice has been to attach the tool to the tool rest by adjusting the tool tip position so that the latter is at the predetermined position fixed as the reference point or to accurately measure the position of the tool tip after the attachment of the tool, feed the measured value into the control section and compute the amount of movement of the tool rest from said position, thereby controlling the movement of the tool rest for machining the workpiece to a desired shape. The amount of movement of the tool tip position is computed and detected by producing pulse signals at the rate of one pulse for every predetermined amount of movement (for example, 0.001 mm) by a rotary position detector installed at the end of a feed screw for the tool rest driven by a servo-motor, or by a linear scale attached to a slide. The distance between the tool tip position and the machining reference point must be permanently stored. For this reason, non-volatile memory such as core memory is employed or if volatile memory such as semiconductor memory is used, battery support is incorporated to prevent disappearance of the stored data when power is cut off. Even if such measures are taken for the memory to retain the stored data when power is cut off, however, the other circits will stop operating when power is cut off. If, therefore, the tool rest is moved during cutting off of power by some external cause, such as vibrations and external forces, or by reason of inertia in the case of a power breakdown taking place in the course of machining, the amount of movement will not be fed into the memory, so that if machining is restarted under such conditions when power is restored, said accidental movement causes errors. To avoid this, it might be contemplated to battery-support the greater part of the memory to update the contents of the memory by detecting the amount of accidental movement of the tool rest during cutting off of power. However, battery-supporting all the control circuitry would require a very large battery capacity because of high power consumption and would be practically impossible.